Dual-GPU cards always list the combined memory, which means a 6GB 7990 is the equivalent of two 3GB 7970's (more or less, depending on the clock speeds in use). So a "6GB" 7990 has twice the rendering power and half the actual memory as a 6GB 7970.

The cases were 6GB per GPU would be better than 3GB per GPU are few and far between. If you don't have a 30" monitor or several smaller monitors in an Eyefinity setup, you won't encounter any of those cases.